A substrate having a transparent electrode that is used for a device such as a touch panel, a display, or a solar cell widely uses a compound such as indium tin oxide (ITO), tin oxide, or zinc oxide as a transparent electroconductive layer. It is known that such a transparent electroconductive layer may be formed not only by a physical vapor deposition method (PVD method), such as a magnetron sputtering method, a molecular beam epitaxy method, and/or a chemical vapor deposition method (CVD method), such as a thermal CVD method or a plasma CVD method, but also by an electroless method.
ITO excels among transparent electroconductive materials, and recently has been widely used in a transparent electroconductive layer. However, because of the possibility of depletion of the raw material indium, a search for alternative materials to ITO has resulted, which requires urgent attention to both resources and costs.
Zinc oxide (ZnO) may be put forth as an example of transparent electroconductive materials alternative to ITO. It is said that ZnO is more transparent but is lower in water and heat stability than ITO (see Nonpatent Document 1 specified below).
Patent Documents 1 and 2, specified below, describe that a transparent electrode containing ZnO combined with chrome or cobalt and group III or group IV atoms has good etching characteristics.
On the other hand, Patent Document 3, specified below, describes that a transparent electroconductive body formed by lamination of a hard-coat layer on a plastic base and further lamination of a transparent electroconductive layer on the hard-coat layer, the transparent electroconductive layer being composed mainly of zinc oxide doped with 4 to 6 atom % silicon, has a favorable sheet resistance stability (i.e., a small change of resistance in reliability tests). However, the transparent electroconductive body described in Patent Document 3 may have lower electrical conductivity because it contains about 5 atom % silicon. In contrast, reduction of the silicon content to improve the electrical conductivity results in decreased stability. Further, it is reported that improved chemical stability may be achieved with a protective film is provided by treating a surface of a zinc oxide transparent electroconductive oxide with an aqueous solution containing triad cations (see Patent Document 4 specified below).
Zinc oxide is a compound having a high ionic property, and among thin-film materials is thus sensitive to water or chemicals. A first approach to compensate for such weakness is to intercept water by a covering layer formed on a surface of a zinc oxide transparent thin film. Materials such as metals or polyolefins can generally function as water barriers, but most of them may be ill-suited for use with a substrate having a transparent electrode because they are nontransparent materials or insulators. A second approach to provide stability is by doping the zinc oxide. In the descriptions of Patent Documents 1 to 3, doping with a substance such as cobalt, chrome, and silicon can improve stability.
These approaches can be applied by a method, such as film-formation, after mixing zinc oxide with another metal oxide or a metal chloride, or by another method, such as co-sputtering zinc oxide with silicon dioxide. Further, JP H10-237630 A (Patent Document 5, specified below) describes a way to form a transparent electroconductive layer by a sputtering method using a metal target in a carbon-containing gas, such as carbon dioxide. However, productivity problems may arise with this approach because the zinc metal used as the metal target may be easily oxidized, causing poor stability of the target's composition.
JP S62-154411 A (Patent Document 6, specified below) discloses a transparent electroconductive film composed mainly of zinc oxide and containing group IV elements, such as silicon. Patent Document 6 relates to the same application as JP H5-6766 B, which is described in Paragraph [0004] in Patent Document 3. The transparent electroconductive film relating to Patent Document 6 is disadvantageous in that electrical conductivity of the electroconductive film is decreased if left in a high temperature and high humidity environment because of age instability of an electric resistance value, which is specified in the paragraphs [0005] to [0007] in the Patent Documents 3. Herein, in an embodiment in Patent Document 6, a film-formation is performed at a power density of 2 W/cm2 in the case of sputtering.
JP 2002-217429 A (Patent Document 7, specified below) reports on zinc oxide doped with aluminum, gallium, boron, or indium in addition to silicon in relation to a translucent electroconductive film used in a photoelectric conversion element. However, although the translucent electroconductive film is formed by sputtering, a target used for the sputtering contains more than two kinds of elements other than oxygen. Such a target requires very difficult processes to mix uniformly and to sinter, causing production and/or productivity problems as the area of the target increases.
As described above, although application of ZnO alternatives to ITO to a transparent electroconductive layer has been widely developed, a material superior to the now-commonly used ITO has not been put sufficiently to practical use.
Much discussion has been made over chemical stability, including supposition that oxygen attachment to a crystal grain boundary prevents charge transfer between crystal particles. Elimination of crystal grain boundaries by making amorphous zinc oxide supposedly improves chemical stability. However, zinc oxide is a compound having good crystallinity, and thus, an electroconductive and amorphous zinc oxide transparent electroconductive oxide has rarely been reported, except for IZO, containing indium oxide (Nonpatent Document 2, specified below). However, IZO contains zinc oxide of only about 10 atom % and is composed mainly of indium oxide. As described, an amorphous zinc oxide transparent electroconductive oxide composed mainly of abundant zinc oxide has not been found yet. The Nonpatent Documents 3 and 4, specified below, are documents generally related to sputtering.
Patent Documents
    Patent Document 1: JP 2002-75061 A    Patent Document 2: JP 2002-75062 A    Patent Document 3: JP H8-45352 A    Patent Document 4: JP 2001-39712 A    Patent Document 5: JP H10-237630 A    Patent Document 6: JP S62-154411 A    Patent Document 7: JP 2002-217429 ANonpatent Documents    Nonpatent Document 1: “Tomeidodenmaku” (“Transparent Electroconductive Film”) edited by Mr. Yutaka Sawada, pp. 6-19, 1999, issued by CMC Publishing, Inc.    Nonpatent Document 2: R. Martins, J. Appl. Phys., 101. 1, 2007    Nonpatent Document 3: “Hakumakukeisei-no-Kiso (4th edition)” (“Basic of Thin Film Formation”) authored by Mr. Tatsuo Asamaki, pp. 221-224, 2005, issued by The Nikkan Kogyo Shinbun Ltd. (Business & Technology Daily News)    Nonpatent Document 4: S. Ishibashi et al., J. Vac. Sci. Technol. A. Vol. 8, No. 3 May/June, pp. 1403-1406, 1990